This invention relates to metal filtration. More particularly, it relates to the removal of molten salt inclusions from liquid metal during metal filtration.
The refining of molten aluminum to produce critical aluminum products typically consists of a two stage process. The first stage involves treating the molten aluminum in a degassing system with a mixture of an inert carrier gas, which is usually argon, along with chlorine, primarily for the removal of hydrogen, soluble alkaline metal impurities and certain large inclusions which are normally present in the molten aluminum. The process for removing hydrogen is called degassing. The details of one degassing process is taught in U.S. Pat. No. 4,317,679, which is assigned to SELEE Corporation, assignee of the present invention. In addition, there are other degassing processes.
The second and final stage consists of filtration, typically with a ceramic foam filter, bed filter, or rigid media filter, to remove the remaining solid inclusion particles which exist in the molten aluminum.
To obtain optimum quality requires careful control of the chlorine levels in the degasser in order to obtain adequate alkaline metal and hydrogen removal. If excess chlorine is used beyond that required for trace alkaline metal and hydrogen removal, then the potential exists for excess liquid salt generation, which principally is magnesium chloride. That is, if excess chlorine is used beyond that required for the removal of the trace alkaline elements and hydrogen, then excessive quantities of liquid magnesium chloride inclusions will be generated.
In addition, other salts or mixtures of salts, including chlorides and fluorides, may be formed.
A well designed, maintained and properly operating degassing system is capable of generating very small argon gas bubbles. Liquid salt droplets occur on the surface of the argon gas bubbles and can be sheared from the surface of the argon bubbles. The size of the liquid salt droplets may range from 1 to 50 microns, and, even though the salt droplets are significantly less dense than molten aluminum, they tend to remain suspended in the molten aluminum due to their size. The suspended salt droplets, over time, tend to grow because of collisions with other droplets and coalescence. These liquid salt particles cannot be reliably removed with conventional filtration technology and can result in oxide patches which create problems for the final aluminum product. Small magnesium chloride droplet inclusions can result in surface patches of oxides of aluminum and magnesium which can result in edge cracking during the subsequent rolling of the aluminum.
One approach to this problem is to monitor the amount of liquid salt inclusions in the molten metal and to use this information to control the degasser. One such monitor system is described in U.S. Pat. No. 5,569,845, assigned to SELEE Corporation, assignee of the present invention.
While the monitor and control approach represents an advance in the art, it is desirable to remove any liquid salt inclusions which may be present in the molten aluminum. The presence of the liquid salt can have a significant impact on the reliability and efficiency of the solid inclusion filter, and can result in significant deterioration in the performance of the ceramic foam filter. As little as 100 parts per million ("ppm") of liquid salt can reduce the capture efficiency of a ceramic foam filter from a range of 70% to 80% to a range of 25% to 40%. It is believed that this reduction in filtration efficiency is caused by the liquid salt coating the inclusion particles, which significantly alters the interfacial surface energy relationship between the inclusions, the molten aluminum and the filter.
Technology currently exists which attempts to separate liquid salts from liquid aluminum by coalescing the small salt particles and removing the coalesced liquid salt by floatation. U.S. Pat. Nos. 4,390,364, 5,122,184 and 5,336,295 deal with the removal of liquid salts by the use of ceramic foam or packed beds as a coalescent medium in order to enhance separation by floatation. U.S. Pat. No. 4,790,873 shows a staged filtration system where the upward flow through the first stage filter serves to coalesce fine salt particles and subsequently release them as large conglomerates which will more readily separate by floatation.